The invention relates to the field of applying protective coatings, and in particular to plasma discharge, for example plasma-electrolytic oxidation, coating of articles made of metals and alloys. This process makes it possible rapidly and efficiently to form wear-resistant, corrosion-resistant, heat-resistant, dielectric uniformly-coloured ceramic coatings on the surfaces of these articles.
The coatings are characterised by a high degree of uniformity of thickness, low surface roughness and the virtual absence of an external porous layer, the removal of which usually involves considerable expense in conventional coating processes.
The process for producing the coatings and the device for implementing the process, described in this application, can be used in engineering, the aircraft and motor vehicle industries, the petrochemical and textiles industries, electronics, medicine and the production of household goods.
A process for producing a ceramic coating using industrial-frequency 50-60 Hz current is known from WO 99/3 1303. The process enables hard coatings of thickness up to 200 μm, well-bonded to the substrate, to be formed on the surface of articles made from aluminium alloys.
The main problem with this process is the formation of a considerable external porous layer of low microhardness and with numerous micro- and macro-defects (pores, micro-cracks, flaky patches). The thickness of the defective layer amounts to 25-55% of the total thickness of the ceramic coating, depending on the chemical composition of the alloy being processed and on the electrolysis regimes.
Expensive precision equipment is used to remove the porous layer. If the article is of complex shape, with surfaces that are difficult for abrasive and diamond tools to reach, the problem of removing the defective layer becomes difficult to solve. This limits the range of application of the process.
Other problems with the known process are the relatively low rate at which the coating forms and the high energy consumption. It is not possible to increase the productivity of the oxidation process simply by raising the current density to higher than 20 A/dm2, since the process then becomes an arc process rather than a spark one; and due to the appearance of strong local burn-through discharges, the whole coating becomes very porous and flaky and adhesion to the substrate deteriorates.
With the aim of intensifying the oxidation process and improving the characteristics of the ceramic coatings, many researchers have tried to improve the electrolysis pulse regimes, proposing different forms and durations of current or voltage pulses.
A process for forming ceramic coatings where the current has a modified sine wave form is known from U.S. Pat. No. 5,616,229. This form of current reduces heat stresses in forming the ceramic layer and enables coatings of thickness up to 300 μm to be applied. However, industrial frequency current is used in this process, which leads to the formation of a relatively thick external porous layer with high surface roughness and relatively high energy costs.
There is another known process, RU 2077612, for oxidising valve metals and alloys in a pulsed anode-cathode regime, in which positive and negative pulses of a special complex form alternate. The duration of the pulses and of the pause between a positive pulse and a negative one is 100-130 μsec, and the succession frequency is 50 Hz. In the first 5-7 μsec, the current reaches its maximum (up to 800 A/dm2), after which it remains constant for 25-50 μsec. In this case, the shorter pulses and far greater pulse powers enable the discharge ignition time to be reduced considerably, and the main reasons for the formation of the defective outer layers are eliminated. However, the pairs of powerful pulses alternate with unjustifiably long pauses, which leads to a low coating formation rate.
There is also a known process, SU 1767043, for producing oxide coatings in an alkaline electrolyte using positive pulses of voltage, amplitude 100-1000V. These pulses have a two-stage form. Initially, for 1-3 μsec, the voltage rises to maximum, and then falls to about a tenth of this, continuing at a constant level for 10-20 μsec. However, the use of positive pulses alone does not make it possible to produce good-quality coatings with high microhardness and wear resistance.
The closest prior art to the proposed invention is the process described in RU 2070942 for oxidation using alternating positive and negative pulses of voltage, amplitude 100-500V and duration 1-10 μsec, during which, at each of the anode half-periods, high-voltage positive pulses, amplitude 600-1000V and duration 0.1-1 μsec, are also applied. When the pulses are applied, the total current at that moment rises, which creates favourable conditions for discharges. The problem with this process is the use of very short high-voltage positive pulses, which does not make it possible to create discharges of sufficient power. This leads to low productivity of the process, and it is also extremely difficult to implement the proposed process technically for industrial purposes.
While certain novel features of this invention shown and described below are pointed out in the annexed claims, the invention is not intended to be limited to the details specified, since a person of ordinary skill in the relevant art will understand that various omissions, modifications, substitutions and changes in the forms and details of the device illustrated and in its operation may be made without departing in any way from the spirit of the present invention. No feature of the invention is critical or essential unless it is expressly stated as being “critical” or “essential.”